One solution for solving this problem was mentioned by Bartik et al.: Organometallics (1993) 12 164-170, J. Organometal. Chem. (1994) 480 15-21, and by Beller et al.: J. Molecular Catal. A: Chemical (1999) 143 31-39. It consists in carrying out hydroformylation in the presence of an aqueous solution containing a cobalt complex made hydrosoluble by the presence of a phosphine-sulfonate ligand, such as the sodium salt of trisulfonated triphenylphosphine or of a trisulfonated tris-(alkylphenyl)-phosphine. The organic phase containing the aldehydes is thus readily separated from the aqueous phase containing the catalyst.
Despite the significance of these various systems, the low capacity of water to dissolve some organic substrates such as long-chain olefins is a major limitation for these methods. Hydroformylation of this type of feeds very often leads to low reaction rates, making any industrial application unthinkable. Furthermore, water is a very coordinating protic solvent that can be reactive to catalysts. Although it is of great interest, the use of water as the reaction solvent cannot be generalized to all catalyst and substrate types.
It is described in U.S. Pat. No. 5,874,638 filed by the applicant that some limitations linked with the use of water as the reaction solvent (in particular the solubility of long olefins) can be overcome by dissolving some catalytic compounds of transition metals from groups 8, 9 and 10, known for catalyzing hydroformylation, in non-aqueous ionic liquids consisting of organic-inorganic salts liquid at ambient temperature.
However, when the catalyst comprises a cobalt salt or complex, it is very difficult to prevent at least partial formation of dicobalt octacarbonyl and/or cobalt tetracarbonyl hydride under the conditions of the hydroformylation reaction. These two compounds being soluble in the organic reaction phase consisting of at least the olefinic reactant and the aldehydes produced, cobalt recycling by means of the non-aqueous ionic liquid phase is only partial, which leads to catalyst losses.
Besides, it has been shown in U.S. Pat. No. 6,617,474 filed by the applicant that it is possible to increase the reaction rates by carrying out the reaction in an ionic liquid that is partly or entirely miscible with the reaction products, while keeping the advantage of the separation and re-use of the ionic liquid containing the catalyst and improving the recovery of the reaction products, by injecting after the reaction section an organic solvent, weakly or non-miscible with the ionic liquid, which can advantageously be the olefinically unsaturated compound to be hydroformylated and which improves demixing of the reaction effluent products.
In this context, it has been found and described in patent application US-A-2003/0,225,303 filed by the applicant that, in the hydroformylation reaction catalyzed by cobalt complexes used in a non-aqueous ionic liquid, recycling the metal in the ionic liquid is greatly improved by the use of a ligand selected from among the Lewis bases and simultaneously by means of an intermediate depressurization stage between the reaction stage under pressure and the stage of phase separation by decantation. At the end of this depressurization stage, the organic phase is separated in the decantation stage and the non-aqueous ionic liquid phase containing the catalyst can be re-used.
It has now been found that it is possible to greatly improve the reaction rate, on the one hand, and the retention and recycle of the cobalt-based catalyst in the ionic liquid phase, on the other hand, by addition of the ligand in a post-reaction stage. The present invention thus provides a new implementation of the system, notably intended to combine a high reaction rate and improved catalyst retention and recycle.